1. Field of the Invention
The embodiments of the invention relates to a display device, and more particularly, to a dual panel type organic electroluminescent display (ELD) device. Although embodiments of the invention are suitable for a wide scope of applications, it is particularly suitable for increasing brightness of an organic electroluminescent display device.
2. Discussion of the Related Art
In general, an organic ELD emits light by injecting electrons from a cathode and holes from an anode into an emission layer such that the electrons combine with the holes to generate an exciton, which then transitions from an excited state to a ground state. In comparison to a liquid crystal display (LCD) device, an additional light source is not necessary for the organic ELD to emit light because the transition of the exciton between the excited state and the ground state causes light to be emitted. Accordingly, the size and weight of the organic ELD is less than an LCD device. The organic ELD has other excellent characteristics, such as low power consumption, superior brightness, and a fast response time. Because of these characteristics, the organic ELD is seen as the display for the next-generation of consumer electronic applications, such as cellular phones, car navigation system (CNS), personal digital assistants (PDA), camcorders, and palmtop computers. Moreover, since fabricating the organic ELD is carried out with fewer processing steps, the organic ELD is much cheaper to produce than an LCD device.
Two different types of organic ELDs exist: passive matrix and active matrix. While both the passive matrix organic ELD and the active matrix organic ELD have a simple structure and are formed by a simple fabricating process, the passive matrix organic ELD requires a relatively large amount of power to operate. In addition, the display size of a passive matrix organic ELD is limited by the width and thickness of conductive lines used in the interconnection of the pixels. Further, as the number of conductive lines increases, the aperture ratio of a passive matrix organic ELD decreases. In contrast, active matrix organic ELDs are highly efficient and can produce a high-quality image on a large display with relatively low power.
FIG. 1 is a cross-sectional view of an organic ELD according to the related art. As shown in FIG. 1, an organic ELD 1 includes first and second substrates 12 and 28 facing each other and spaced apart from each other. An array element layer 14, including thin film transistors “T,” is formed on the first substrate 12 Although not shown, the array element layer 14 further includes a gate line, a data line crossing the gate line, and a power line crossing one of the gate and data lines. The data line, gate line and power line define a pixel region “P.” A first electrode 16 is on the array element 14 in each of the pixel regions “P,” an organic electroluminescent (EL) layer 18 is on the first electrode 16 in each of the pixel regions “P,” and a second electrode 20 on the organic EL layer 18 of all of the pixel regions “P.” More specifically, the first electrode 16 of each pixel region “P” is connected to the thin film transistor “T” of each pixel region “P.” The organic EL layer 18 includes red (R), green (G) and blue (B) sub-organic EL layers in the pixel regions “P.”
The second substrate 28 functions as an encapsulating panel and has a recessed portion 21. A desiccant 22 is positioned in the recessed portion 21 to protect the organic ELD 1 from moisture. A seal pattern 26 is formed between the first and second substrates 12 and 28 at a periphery thereof to attach the first and second substrates 12 and 28 to each other.
FIG. 2 is an equivalent circuit diagram of an organic ELD according to the related art. As shown in FIG. 2, a gate line 42 and a data line 44 crossing the gate line 42 are formed on a substrate 32 to define a pixel region “P.” A power line 55 that crosses the data line 44 is parallel to and spaced from the gate line 42.
A switching element “TS” is connected to the gate and data lines 42 and 44 at an adjacent portion of crossing the gate and data lines 42 and 44. A driving element “TD” is connected to the switching element “TS.” For example, the driving element “TD” is a P-type thin film transistor, as shown in FIG. 2. A storage capacitor “CST” is formed between the switching element “Ts” and the driving element “TD.” A drain electrode 63 of the driving element “TD” is connected to a first electrode (not shown) of an organic EL diode “E.” The source electrode 66 of the driving element “TD” is connected to the power line 55.
Hereinafter, an operation characteristic of the organic ELD will be explained in detail as follows. When a gate signal is applied to a gate electrode 46 of the switching element “Ts,” a current signal applied to the data line 44 is changed into a voltage signal through the switching element “Ts” and is applied to the gate electrode 68 of the driving element “TD.” As a result, the driving element “TD” is driven and the level of the current applied to the organic EL diode “E” is determined. And then, the organic EL diode “E” can embody gray scale depending on the level of the current applied to the organic EL diode. Because the signal in the storage capacitor “Cst” acts to maintain the signal of the gate electrode 68 of driving element “TD,” the current level applied to the EL diode can be maintained until the next signal is applied even if the switching element “Ts” is in the OFF state.
FIG. 3 is a plan view of an organic ELD in one pixel region according to the related art. As shown in FIG. 3, a switching element “Ts,” a driving element “TD” connected to the switching element “Ts,” and a storage capacitor “Cst” are formed on a substrate 32 in a pixel region “P.” For example, the substrate 32 includes a transparent insulating substrate, such as glass or plastic. Alternatively, the functions of the switching element “Ts” and the driving element “TD” can be implemented with more than two switching elements in the pixel region “P.”
A gate line 42 is formed on the substrate 32 and a data line 44 crosses the gate line 42 to define the pixel region “P.” In addition, a power line 55 parallel to the data line 44 crosses the gate line 42. The switching element “Ts” includes a first gate electrode 46 connected to the gate line 42, a first semiconductor layer 50 over the first gate electrode 46, a first source electrode 56 connected to the data line 44, and a first drain electrode 60 spaced apart from the first source electrode 56. The driving element “TD” includes a second gate electrode 68 connected to the first drain electrode 60, a second semiconductor layer 62 over the second gate electrode 68, a second source electrode 66 connected to the power line 55, and a second drain electrode 63. More particularly, the first drain electrode 60 and the second gate electrode 68 can be connected to each other via a contact hole 64 in an insulating material layer (not shown). Further, a first electrode 36 is connected to the second drain electrode 63 in the pixel region “P.”Although not shown, a storage capacitor “Cst” includes a first storage electrode of doped silicon, a second storage electrode that is a portion of the power line 55, and an insulating material layer between the first storage electrode and the second storage electrode.
FIG. 4 is a cross-sectional view of an organic ELD along line “IV-IV” in FIG. 3. As shown in FIG. 4, the second semiconductor layer 62 is formed on the substrate 32, a gate insulating layer “GI” is formed on the second semiconductor layer 62, the second gate electrode 68 is formed on the gate insulating layer “GI” over the second semiconductor layer 62, and an interlayer insulating layer “IL” is formed on the second gate electrode 68, which includes first and second contact holes “C1” and “C2” that expose both end portions of the second semiconductor layer 62. The second source and second drain electrodes 66 and 63 are formed on the interlayer insulating layer “IL” and are connected to the second semiconductor layer 62 via the first and second contact holes “C1” and “C2.” A passivation layer 68 is formed on the second source and second drain electrodes 66 and 63. The passivation layer 68 also includes a drain contact hole “C3” that exposes a portion of the second drain electrode 63. The first electrode 36 is connected to the second drain electrode 63 via the drain contact hole “C3.” The organic EL layer 38 is formed on the first electrode 36, and a second electrode 80 is formed on the organic EL layer 38. The first electrode 36 and the second electrode 80 are a cathode and an anode, respectively. The first electrode 36, the organic EL layer 38, and the second electrode 80 constitute the organic EL diode “E.” If the driving element “TD” is an N-type TFT, the first electrode 36 and the second electrode 80 are a cathode and an anode, respectively. On the other hand, if the driving element “TD” is a P-type TFT, the first electrode 36 and the second electrode 80 are an anode and a cathode, respectively.
The storage capacitor “Cst” and the driving element “TD” are disposed in a row. The second source electrode 66 is connected to the second storage electrode 55. The first storage electrode 35 is disposed under the second storage electrode 55.
FIG. 5 is a cross-sectional view of an emission region of an organic ELD according to the related art. As shown in FIG. 5, the emission region of the organic ELD 1 includes an anode 36 on the substrate 32, a hole injection layer 38a on the anode 36, a hole transport layer 38b on the hole injection layer 38a, an emitting layer 38c on the hole transport layer 38b, an electron transport layer 38d on the emitting layer 38c, an electron injection layer 38e on the electron transport layer 38d, and a cathode 80 on the electron injection layer 38e.The hole transport layer 38b and the electron transport layer 38d act to transport holes and electrons to the emitting layer 38c to improve light emitting efficiency. Further, the hole injection layer 38c between the anode 36 and the hole transport layer 38b reduces hole injecting energy, and the electron injection layer 38e between the cathode 80 and the electron transport layer 38d reduces electron injecting energy so as to increase light emitting efficiency and reduce the driving voltage.
The cathode 80 can be made of one of calcium (Ca), aluminum (Al), magnesium (Mg), silver (Ag), and lithium (Li). The anode 36 can be made of a transparent conductive material, such as indium tin oxide (ITO). Because the anode 36 is a transparent conductive material, such as ITO, which can be deposited by sputtering, layers under the anode 36, such as the emitting layer 38, may be damaged during the sputtering to form the anode 36. To prevent damage to the emitting layer 38 during sputtering of the anode, the anode 36 is not formed on the emitting layer 38 but rather the emitting layer 38 is formed on the anode 36.
When light from the emitting layer 38 is emitted toward the anode 36 formed under the emitting layer 38, the aperture region is limited due to the array element (not shown) under the anode 36. Consequently, because the above-described organic ELD according to the related art is a bottom emission type device, brightness is deteriorated due to the array element reducing the aperture regions of the device. Further, to prevent minimization of the aperture regions, the design of the array element is limited in a bottom emission type device. Furthermore, the driving element is typically P-type, which has a complicated fabrication process that reduces product yield.